The invention relates to a secondary coating line for producing a secondary-coated optical fibre or bundle of fibres, the line comprising a feeding apparatus with a feed roll for feeding an optical fibre or bundle of fibres; an extruder for extruding an oversize secondary jacket around the fibre or bundle of fibres; a cooling apparatus for cooling the extruded secondary jacket, the distance between the extruder and the point where the cooling apparatus starts to cool being adjustable; and a winding apparatus for winding the finished secondary-coated fibre or bundle of fibres on a take-up roll.
A conventional secondary coating line with the basic structure described above is known e.g. from Finnish Patent 75,939, or corresponding British Patent Application 2,141,557. Conventionally, secondary coating lines like this have been controlled so that a product produced during the starting and speeding up of the line has not met the quality requirements. The reason for this is that when the speed of the line changes, the characteristics of the produced secondary coating also change; not until the normal running speed is achieved does the secondary coating reach the desired values. A rather large amount of valuable optical fibre has thus been wasted.
Alternatively, optical fibres have not been thread into the tube provided by the secondary jacket until the production speed has been achieved. Plastic waste has been produced, and the production speed must have been adjusted to be so slow that it has been possible to thread the optical fibres into the secondary jacket at that rate.
When optical fibres are thread into the secondary jacket at a rate that is slower than the normal running speed, the poor quality of the product is at least primarily due to the fact that the distance between the cooling apparatus and the extruders has not been suitable considering the rate. In practice, the plastic melt outlet from the press head forms after the press head a conical part in which the secondary jacket tube is drawn to its final measurements. If the water cooling conducted in the cooling apparatus is started at the conical part, the result is a brittle tube, i.e. the tensile strength of the tube in a tension test is small. If on the other hand the distance between the press head and the water cooling is too long, the hot plastic may run, causing faults in shape and variation in diameter. As the line speed increases, the tension cone following the press head becomes larger; by suitably adjusting the line speed, it has thus been possible to make the measurements of the secondary jacket comply with the quality requirements.
In secondary coating, the type of plastic from which a secondary jacket is produced must also be taken into account. Since different plastic types have different characteristics, they behave differently in an extruder, and so the plastic melt also behaves differently and the line speed must be suitable in order that it might be possible to achieve the desired measurements.